New Criteria for 0D/1D Knock Models to Predict the Knock Boundary for Different Gasoline Fuels 2021-01-0377

As engine knock limits the efficiency of spark ignition engines and consequently further reduction of CO₂ emissions, SI engines are typically designed to operate at the knock boundary. Therefore, a precise knock model is necessary to consider this phenomenon in an engine process simulation. The basis of the introduced 0D/1D knock model is to predict when the unburnt mixture auto-ignites, since auto-ignitions precede knocking events. The knock model further needs to evaluate the auto-ignition, because not every auto-ignition results in engine knock. As the introduced model’s prediction of the auto-ignition onset is already validated at extensive variations of operating conditions, this publication focusses on its evaluation. For this, two new, independent criteria are developed that take the pre-reactions of the unburnt mixture before the start of combustion into account to calculate a respective threshold for the auto-ignition onset at the knock boundary. In simulations, the operating point’s center of combustion is advanced until the 0D/1D knock model predicts an auto-ignition that occurs before its calculated threshold. For validation, the point where 50% of the mass is burned (MFB50) of this simulated operating point is compared to the MFB50 of the corresponding measured average working cycle at the knock boundary. To ensure a wide-ranging validation of the model, the measurement data cover extensively varied operating conditions like different engine configurations and gasoline fuels, varying compression ratio, turbulence, injection, EGR, wall temperature as well as engine load and speed. Although each introduced criterion is calibrated at only two operating points, the respective new 0D/1D knock model can predict the MFB50 at the knock boundary of the stated broad variations very precisely with a standard deviation below 2 °CA. Consequently, both new, independent criteria clearly improve the knock model and help to improve the development of SI engines in 0D/1D simulation.